1. Field of the Invention
The present invention relates to a nickel base alloy which is substantially free of rhenium yet at the same time achieves the creep resistance properties of the second-generation nickel base superalloys.
2. Discussion of Background Information
In gas turbines, such as fixed gas turbines or aero engines, nickel base superalloys are used, for example, as blade materials, since even at the high operating temperatures these materials still have sufficient strength for the high mechanical loads. For example, in fixed gas turbines or jet engines in commercial aircraft, turbine blades are exposed to a stream of exhaust gas at temperatures of up to 1500° C. and at the same time are subject to very high mechanical loads as a result of centrifugal forces. Under these conditions it is particularly important for the creep resistance of the material used to meet the requirements. To raise the creep resistance further, turbine blades have for a number of decades also been produced in monocrystalline form, in order, through the avoidance of grain boundaries, to achieve further improvement in the creep resistance.
With the so-called second-generation and third-generation nickel base superalloys that are presently in use, the alloys typically include the chemical element rhenium, with a fraction of three or six percent by weight, since rhenium further improves the creep resistance.
However, in view of the scant availability of rhenium, the admixing of rhenium is very expensive. In the prior art, accordingly, there have already been efforts to reduce the fraction of rhenium or to do without the alloying of rhenium entirely, while at the same time maintaining the mechanical properties, especially with regard to the creep resistance. Relevant studies include those by A. Heckl, S. Neumeier, M. Göken, R. F. Singer, “The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys”, in Material Science and Engineering A 528 (2011) 3435-3444, and by Paul J. Fink, Joshua L. Miller, Douglas G. Konitzer, “Rhenium Reduction—Alloy Design Using an Economically Strategic Element”, JOM, 62(2010), 55-57. Furthermore, such alloys are also subject matter of patent applications, as for example in EP 2 305 847 A1, EP 2 305 848 A1, EP 2 314 727 A1, US 2010/0135846 A1, WO 2009/032578 A1, and WO 2009/032579 A1. The entire disclosures of the documents mentioned above are incorporated by reference herein.
Although, therefore, there have already been some proposed solutions for a reduction in rhenium or for rhenium-free nickel base superalloys, there continues to be a need to develop reduced-rhenium or rhenium-free nickel base superalloys whose mechanical properties, especially high-temperature properties, such as creep resistance, are situated within the range of the rhenium-containing nickel base superalloys presently in use, or which avoid the use of certain elements such as hafnium.
It is desirable, therefore, to provide a nickel base superalloy which has comparable mechanical properties, especially high-temperature properties, such as creep resistance, with second-generation and third-generation nickel base superalloys that are currently in use, but which does away entirely with the alloying of the element rhenium. Furthermore, the alloy is to be capable of production economically and efficiently, and more particularly is to be readily castable and also monocrystalline or amenable to directional solidification.